This invention relates to a controlled electronic switching device for the suppression of transients, which can change over between two different (on and off) states according to the state of an externally applied control signal.
Such devices have several applications in the field of either current or voltage protection, e.g. in telephone lines.
The need is felt for devices which can be arranged on the line between the two leads so as to suppress transient phenomena, such as the secondary effects of lightning, by operating as open circuits in a normal condition and shorting the two lines together in the presence of a transient phenomenon ("normalized" lightning) to be suppressed.
Suitable devices to provide this function are, for example, SCR switches formed with four layers of alternate conductivity types, as the one shown for instance in FIGS. 1 to 3. Such a device (well known per se) comprises two terminal connections, an anode A, cathode K, and a control electrode or gate G, and has three junctions J.sub.1,J.sub.2,J.sub.3 which govern the device behaviour. In particular, by negatively biasing the cathode with respect to the anode, the junctions J.sub.1 and J.sub.3 become forward biased while junction J.sub.2 is inversely biased; consequently, there is no current flow between the anode and cathode, and the device will be in its off state.
On application of a signal to the control electrode G, the NPN transistor formed by the three layers closest to the cathode begins to conduct, thus lowering its collector voltage level which corresponds, as shown, to the base of the second PNP transistor formed by the three layers closest to the anode A. As a result, the second PNP transistor senses that occurrence as a base drive and begins to conduct, thus producing a regenerative effect. By this time, the device will be in its fully conductive state as represented by the vertical line in the graph of FIG. 3 which also illustrates the relationship existing between current and voltage versus the control current to the electrode G. As is known, (refer, for instance, to Gentry et al., "Semiconductor Controlled Rectifiers: Principles and Application of p-n-p-n Devices", Prentice-Hall, E-E Series) in order for the device to turn on the following condition must be met: .beta..sub.NPN .times..beta..sub.PNP =1. As the current I.sub.A flowing through the device decreases, the device remains on in accordance with the law illustrated in FIG. 3, until the current reaches a minimum value called holding current, I.sub.H.
However, that prior device has the disadvantage of becoming conductive as the voltage across it varies rapidly even with the control current to the electrode G below the desired value for conduction to begin. This phenomenon, which is due to the appearance of capacitance at the junctions, has been obviated by the device, also well known, shown in FIGS. 4 and 5. The technique adopted (refer, for example, to R. W. Aldrich and N. Holonyar Jr. in the article entitled "Two-terminal Asymmetrical and Symmetrical Silicon Negative Resistance Switches", Journal of Applied Physics, Vol. 30, No. 11, November, 1959) consists in practice in providing a resistance between the base region of the NPN transistor and the emitter thereof, which diverts the current generated within the capacitor owing to a voltage variation. In particular, and as shown in FIG. 4, on a silicon chip comprising four layers with different conductivity types, namely a layer 1 of the P.sup.++ type, layer 2 of the N type, layer 3 of the P.sup.+ type, and layer 4 having several regions of the N.sup.+ type, a metal layer 5 has been deposited which part overlaps the layers 3 and 4. The circuit equivalent of such a device is shown in FIG. 5. In that view, one can see the transistor 7 of the PNP type formed by the layers 1, 2, and 3 of FIG. 4, the transistor 8 formed by the layers 2,3 and 4 of the same, the capacitor 10 formed between the layers 2 and 3 of FIG. 4 (and corresponding to the junction J.sub.2), and the resistor 9 placed between the emitter and base of the transistor 8 and due to layer 5. In particular, the resistance of element 9 determines the value of the triggering current of the gate electrode G.sub.1. That resistance, which is selected at a very low value to avoid the capacitive current from the capacitor 10 causing the device to turn on prematurely, also causes the control current from the gate electrode G.sub.1 to only turn on the device when relatively high. In particular, by selecting the value of 1.OMEGA. for R, the triggering current of the gate electrode is fixed at 600-700 mA for the device to turn on.